BANGKOK (Reuters) - Thai Prime Minister Prayuth Chan-ocha vigorously defended his position as leader on Wednesday, more than six months after he seized power in a bloodless coup, as a U.S.-based rights group said the country had fallen into an "apparently bottomless pit".

Thailand has seen a fresh wave of resistance to the junta over the past week with dozens detained and arrested for flashing anti-coup signs in public at the prime minister and for distributing anti-coup leaflets in the capital, Bangkok.

Critics of the coup had largely gone to ground following an army crackdown aimed at silencing dissenters. Those who express disagreement face a possible two-year prison term.

Prayuth, who as army chief seized power from an elected government in May and was appointed prime minister in August, defended his role as prime minister and vowed not to use force against dissenters.

"I did not seize power for my benefit. We do not want to abuse power and we do not want to use force," said Prayuth, who is also head of the junta, formally known as the National Council for Peace and Order (NCPO).

"My being in this position has not damaged the country."

The army took power after months of street protests that helped to oust the government of Prime Minister Yingluck Shinawatra. The coup was widely condemned by the West.

As part of its crackdown, the junta has also vowed to ramp up prosecutions of critics of the monarchy.

In a rare move, three senior policemen were charged this week with violating Thailand's strict lese-majeste laws which make it a crime to defame, insult or threaten the king, queen, heir to the throne or regent.

The New York-based Human Rights Watch said in a statement that Thailand had "fallen into an apparently bottomless pit".

"Six months after the coup, criticism is systematically prosecuted, political activity is banned, media is censored, and dissidents are tried in military courts," it said.

Thailand remains under martial law and all political gatherings of more than five people are banned. Violators risk trial in military courts.